1. Field of the Invention
The present invention relates to hydraulic pumps and, more particularly, to hydraulic pumps suitable for use in a vehicular power steering application.
2. Description of the Related Art
Many modern vehicles have hydraulic power assisted steering systems having a power steering pump for circulating the hydraulic fluid within the system. The power steering circuit typically includes a reservoir from which hydraulic fluid is fed to the pump. Fluid discharged by the pump is used to operate a steering gear and then returns to the reservoir. The reservoir not only collects hydraulic fluid for intake by the pump but also typically conditions the hydraulic fluid by de-aerating and filtering the hydraulic fluid. The reservoir may also act as a thermal sink and cool the fluid. Fluid from the reservoir is fed to the pump and the cycle is repeated. The pump will often generate more fluid flow than is necessary for operation of the steering gear and the pump will typically include a flow control valve that re-circulates excess flow from the discharge to the intake channels of the pump.
FIG. 1 presents a simplified schematic diagram of a power steering system 20 for a vehicle. Power steering system 20 includes a pump 22 which discharges fluid into a hydraulic line 24 leading to steering gear 26. Hydraulic fluid flows through the circuit from steering gear 26 to reservoir 28 where it is de-aerated and filtered prior to returning to pump 22. A flow control valve 30 diverts fluid from discharge line 24 of pump 22 to intake line 32 of pump 22 when pump 22 is producing excessive fluid flow. Although flow control valve 30 is presented as an external valve in the schematic depiction shown in FIG. 1, valve 30 is physically located within the housing of pump 22.
For many vehicles, the power steering pump shaft typically has a pulley that is driven by a belt that is also coupled to a pulley on the vehicle crankshaft. It is also known to drive the power steering pump by an electrical motor.
A variety of different types of power steering pumps are known in the art and four general types of pumps that can be used for such power steering pumps include vane, roller, slipper and gear pumps. Vane-type steering pumps are in common use in contemporary vehicles and examples of vane-type steering pumps are disclosed in U.S. Pat. No. 6,913,446 B2 issued to Nissen et al.; U.S. Pat. No. 6,899,528 B2 issued to Youngpeter et al.; U.S. Pat. No. 6,857,863 B1 issued to Modrzejewski et al.; and U.S. Pat. No. 6,666,670 B1 issued to Hartman et al., the disclosures of which are hereby incorporated herein by reference.
FIG. 2 is a simplified exploded view of pump 22 which is a high flow vane pump. Pump 22 includes a housing 34 which in the illustrated embodiment is a cast aluminum housing or other suitable material. A drive shaft 36 extends through housing 34 and thrust plate 38 into interior housing volume 40. A rotor 42 is mounted on shaft 36 within housing volume 40 and includes slots 44 in which vanes 46 are located. A pump rotating group including camming ring 48 is schematically depicted in FIG. 3 and is located within housing volume 40. Camming ring 48 surrounds rotor 42, vanes 46 and chambers 50. (Camming ring 48 is not shown in FIG. 2.) An end plate assembly 52 seals the end of housing volume 40 opposite thrust plate 38 and is secured within housing 34 by a retaining ring 54 or other suitable means. The general operating principles of vane-type pumps such as pump 22 are well known to those having ordinary skill in the art.
FIGS. 4 and 5 illustrate prior art pump 22a, a known, commercialized embodiment of vane-type pump 22. In pump 22a, thrust plate 38 has first and second openings 56, 58 through which hydraulic fluid flows into an intake chamber. Openings 56, 58 are oblong, and each has opposed terminal ends 60. Relative to each opening 56, 58, one terminal end 60 is downstream of the other terminal end 60 relative to fluid flowing through the intake flow channel. It is noted that vanes 46 subdivide that portion of housing volume 40 within camming ring 48 into separate chambers 50 and that first and second thrust plate openings 56, 58 are in communication with two separate chambers 50. When these separate chambers 50 are in communication with openings 56, 58 they function as intake chambers, taking in hydraulic fluid. As shaft 36 rotates rotor 42 and vanes 46, the chambers 50 which had just functioned as intake chambers receiving fluid flowing through openings 56, 58, rotate out of communication with openings 56, 58 and begin to function as discharge chambers. After rotating out of communication with openings 56, 58, chambers 50 are rotated into communication with discharge ports and become progressively smaller thereby increasing the pressure of the fluid and discharging the fluid through the discharge ports.
Pump 22a is configured such that fluid entering a chamber 50 through one of openings 56, 58 is discharged through a discharge port after rotation through an angle of approximately 90 degrees about the axis of shaft 36.
Thrust plate 38 is seated against surface 62 formed by housing 34 and which is best seen in FIG. 5. An intake flow channel 64 is formed in surface 62 and has a generally V-shaped configuration when viewed along the rotational axis of shaft 36 as depicted in FIG. 5. Intake line 32 of pump 22 feeds into flow channel 64 through an opening 66 near the apex of the V-shape of channel 64. The fluid flowing into channel 64 through opening 66 is then divided into two flow channels 68, 70 which form the two separate legs of V-shaped channel 64. Intake channels 68, 70 respectively lead to terminal areas 72, 74 which are in fluid communication with openings 56, 58 in thrust plate 38. Terminal areas 72, 74 of intake channels 68, 70 each have a shape and size that generally conforms to their respective opening 56, 58 in thrust plate 38 such that substantially all of the area of an opening 56, 58 is positioned above or over the respective terminal area 72, 74. In operation, fluid flows from opening 66 through channels 68, 70 and then from terminal areas 72, 74 through openings 56, 58 in thrust plate 38 into separate chambers 50.
While many adequate steering pump designs such as pump 22a are known in the art, a steering pump having improved fluid flow and/or pressure balancing characteristics, which reduce the variability and magnitude of pressures within the pump, and provide noise reduction, increased durability, and the opportunity for cost saving benefits, remains desirable.